ViewState anatomy (translation and Notes)

ViewState not what?


1. ViewState is not the value of the control used to recover postbacks.


This is done automatically by matching the variable name of the control in form. This is valid only for controls that were created before the Load event was loaded.


2. ViewState does not automatically re-create any controls that are created dynamically through code.


3. is not used to hold user information. Simply save the control state on this page and not pass between pages.

What is
ViewState?


ViewState is used to track and save state information for controls. Otherwise, this information may be lost because the values do not return with the form or are not in page HTML at all.

The
ViewState holds the changed control properties in the code, binds to any data of the control through code, and any changes that are triggered by user actions and postback.


ViewState also provides a status pack (StateBag), a special set or dictionary (collection or dictionary) that can be used to save and recover arbitrary objects or values through a key.


ViewState Format


the __viewstate hidden field that is saved in the form. It's Base64 encoded, not encrypted!


But it is also possible to encrypt (set enableViewStateMac to use machine key for hashing)


encryption: Set MachineKey authentication, but this must be set at the machine level and requires more resources, so it is not recommended.


Listing 1:viewstate Machine Hash Disabled


Machine.config or web.config: <pages enableviewstatemac= ' false '/>


page level directive: <% @Page enableviewstatemac= ' false '%>


page level Script CODE:PAGE.ENABLEVIEWSTATEMAC = false;


Listing 2:viewstate encryption is Enabled


machine.config: <machinekey validation= ' 3DES ' validationkey= ' * '/>


where the validationkey must be the same across a web-farm setup


also requires the enableViewStateMac property setting to is true


before rendering, ViewState was saved in the Page.savepagestatetopersistencemedium method,


is restored in the Page.loadpagestatefrompersistancemedium method when the postback is made.


These two methods can be easily rewritten to implement saving ViewState into session. This is suitable for small bandwidth occasions,


such as mobile devices by default are session. The code is as follows:


Listing 3:viewstate Saved in Sessions State


protected Override Object LoadPageStateFromPersistenceMedium ()


{


return session["ViewState"];